Processing digital data

ABSTRACT

An original digital audio signal represents unimpaired audio information. The digital audio signal is compressed and encrypted without substantial impairment to produce a first audio signal. A second, unencrypted, audio signal is produced. The first and second audio signals are combined. Preferably the first audio signal is losslessy compressed. The second audio signal may be an impaired version of the original digital audio signal.

BACKGROUND TO THE INVENTION

1. Field of the Invention

The present invention relates to: a method of processing digital data;apparatus for processing digital data; a computer program for processingdigital data; a data format; a method and apparatus for restoring theprocessed data; and a corresponding program. Some aspects andembodiments of the invention relate to processing audio signals, but itwill be appreciated that in other aspects and embodiments the inventionmay be applied to other data. The other data may be audio/visual data,video data, still image data, a computer program, and multimedia dataamongst other examples.

2. Description of the Prior Art

EP-A-1,189,372 (Matsushita) discloses many techniques for protectingaudio signals from misuse. In one technique, audio is compressed andencrypted before distribution to a user. The user needs a decryption keyto access the audio. The key may be purchased by the user to access theaudio. The audio cannot be sampled by a user until they have purchasedthe key. Other techniques embed an audible watermark in an audio signalto protect it. In one technique, an audio signal is combined with anaudible signal (also referred to as a watermark) according to apredetermined rule. The watermark degrades the audio signal. Thecombination is compressed for transmission to a player. The player candecompress and reproduce the degraded audio signal allowing a user todetermine whether they wish to buy a “key” which allows them to removethe watermark. The watermark is removed by adding to the decompresseddegraded audio signal an equal and opposite audible signal. Thewatermark may be any signal which degrades the audio. The watermark maybe noise. The watermark may be an announcement such as “This music isfor sample playback”.

Co-pending UK patent application 0202737.3 (Sony United Kingdom Limited)filed 6 Feb. 2002 discloses a method of modifying a compressed videobitstream for applying a watermark to the video. The bitstream includesdigital codes representing the compressed video. At least one digitalcode is selected. The code occupies a part of the bitstream which is tocontain at least one watermark code which represents a watermarkperceptible in the information signal. The selected, digital code(s) areremoved from the said part of the bitstream. The watermark code(s) areput in the said part of the bitstream in place of the selected code(s).The number of bits of the selected code(s) removed from the said part ofthe bitstream is greater than or equal to the number of bits of the saidwatermark code(s) put in the said part. The removed selected code(s) areencrypted and appended to an end of the bitstream and/or placed inwatermark user data fields created in the bitstream.

WO 02/51150 discloses a system in which an audio signal is transmittedin the blanking period of a video signal. Compressed video informationand compressed audio information are reproduced from a DVD. The video isdecoded by a computer. The computer encrypts the decoded video. Thecomputer also encrypts the compressed audio. The computer outputs theencrypted video and the encrypted audio via a cable.

U.S. Pat. No. 6,041,160 combines, in a multiplexer, encoded audioinformation with compressed video which has been at least partiallyscrambled.

US-A1-2002/0108043 generates MPEG encoded data which comprises audio andvideo information. A switch has two inputs: one is connected to receivethe MPEG encoded data directly and the other via an encrypting device.The switch selects one or the other input to generate partly encryptedMPEG data.

U.S. Pat. No. 4,266,243 comprises a mixer which combines composite videosignals with scrambled and compressed audio signals. The audio signalsare scrambled in a scrambling device and then the scrambled audio iscompressed in a compressor before being applied to the mixer.

GB-A-2 369 022 (Radioscape Limited) describes the delivery of audiofiles by digital radio,

An incomplete and/or partly corrupted audio file in compressed form, istransmitted as a first data stream by digital radio “in the clear” (i.e.unencrypted). An audio file of n frames is made incomplete by removingn-n frames, leaving m to be transmitted. The file is corrupted bycorrupting m-p of those frames leaving p uncorrupted frames to betransmitted “in the clear”.

A second data stream referred to as a “delta stream” comprising the n-mremoved frames and the m-p original (uncorrupted) frames totalling n-pframes. Those n-p frames are encrypted. In one example those n-pencrypted frames are embedded within extra space allocated within theaudio frames themselves.

The receiver is able to reassemble the original audio file byreinserting the n-m removed frames from the second stream into the firstdata stream and replacing the m-p corrupted frames in the first datastream with the m-p original frames taken from the second stream.

SUMMARY OF THE INVENTION

According to a first aspect of the present invention there is providedmethod of processing a digital audio signal comprising the steps of:

a) providing a digital audio signal representing unimpaired audioinformation;

b) compressing and encrypting the said digital audio signal to produce afirst, compressed and encrypted, audio signal the audio information ofwhich is substantially unimpaired compared to that of the said digitalaudio signal;

c) producing an unencrypted second audio signal; and

d) combining the said first and second audio signals to produce acombined signal comprising the said compressed and encrypted first audiosignal and the unencrypted second audio signal.

The digital audio signal is preferably losslessly compressed but it maybe compressed by a “lossy” process. Thus the first compressed andencrypted audio signal is preferably an unimpaired representation of thedigital audio signal due to the loss less compression, but may be asubstantially unimpaired representation to the extent the lossycompression has resulted in the loss of some data.

Because the first audio signal is (substantially) unimpaired, it isrecoverable simply by separating it from the second audio signal anddecrypting it, which is a relatively straight forward process. It doesnot require reassembling using data from another signal (c.f.Radioscape) which is a relatively complex process.

Because the first audio signal is encrypted it is secure fromunauthorised use.

The second signal is unencrypted and thus can be reproduced easily.

Preferably, the first audio signal is compressed and subsequentlyencrypted. Most preferably, the encryption algorithm used to encrypt thefirst audio signal does not significantly increase the number of bits ofthe first audio signal. A small increase in the number of bits may beacceptable.

The compression reduces the amount of data and the encryption transformsthe compressed data into noise. The first signal then appears to a userto be noise in the combined signal.

The second signal may be an impaired version of the (unencrypted anduncompressed) digital audio signal or a filter audio signal.

Where the second signal is an impaired version of the original digitalaudio signal, a user may listen to the second signal (which is notencrypted) to determine whether they wish to access the original digitalaudio signal. If they do wish to access the original digital audiosignal, that signal can be reproduced from the compressed and encryptedfirst signal. The original digital audio signal is kept secure frommisuse in that the user cannot access it without a decryption key whilstthe impaired second signal can be used by the user without decryption.

The second signal may be an impaired and compressed version of the firstsignal in which case a user needs a suitable decompressor to access theimpaired signal.

A second aspect of the present invention provides, in a systemcomprising a transaction server and at least first and second clients, amethod of transferring a digital signal representing content from thefirst client to the second client, the method comprising the steps of:

using the first client to implement the method of said first aspect ofthe invention to produce the combined signal, and associate anidentifier with the combined signal for identifying the combined signal;

providing, to the transaction server, the identifier and at least onekey for decrypting the encrypted signal and storing, in the transactionserver, the said identifier and the said at least one key;

transferring the combined signal to the second client;

deriving the said identifier associated with the combined signal;

transferring the identifier from the second client to the transactionserver;

subject to predetermined conditions being satisfied, transferring fromthe transaction server to the second client at least one key associatedwith the said identifier, for decrypting the encrypted first signal; and

using the second client to separate the first signal from the secondsignal and to restore the first signal.

According to a third aspect, there is provided, in a system comprisingat least first and second processors, a method of transferring a digitalsignal representing content from the first processor to the secondprocessor the method comprising the steps of:

using the first processor to implement the method of said first aspectof the invention to produce the combined signal and to associate anidentifier with the combined signal for identifying the combined signal;

storing the said identifier;

transferring the combined signal to the second processor;

at the said second processor, deriving the said identifier associatedwith the combined signal;

subject to predetermined conditions being satisfied, transferring to thesecond processor at least one key associated with the said identifier,for decrypting the encrypted first signal; and

using the second processor to separate the first signal from the secondsignal and to reproduce the first signal.

A fourth aspect of the invention provides a method of processing adigital signal comprising the steps of

providing a first digital signal representing first information,

providing a second digital signal, and

embedding the first signal in the second signal by replacing LessSignificant Bits (LSBs) of the second signal by bits of the first signaland retaining the More Significant Bits (SBs) of the second signal,

whereby the first signal occurs as noise in the second signal.

Preferably in the fourth aspect the first signal is a compressed signaland/or an encrypted signal.

A fifth aspect of the invention provides a method of processing adigital signal comprising the steps of

providing a first digital signal representing substantially unimpairedfirst information, the first signal being a compressed and/or encryptedsignal,

providing an unencrypted second digital signal representing secondinformation, and which is compressed according to a compression formathaving auxiliary data space, and

combining the first signal comprising the said substantially unimpairedfirst information with the second signal, embedding at least part of thefirst signal being embedded in the said auxiliary data space of thesecond signal.

In the fifth aspect, part of the first signal may be appended to thesecond signal.

A sixth aspect provides a method of processing a digital signalcomprising the steps of

providing a first digital signal representing first information,

providing a second digital signal, and

embedding the first signal in the second signal by selecting groups of Nsamples and distributing over the N samples of each group correspondingsets of M bits of the first signal, where the ratio M/N is an integerfraction.

In the fourth fifth and sixth aspects, the first signal preferablyrepresents a computer program and the second signal preferablyrepresents an audio signal. Thus in an example of the fourth, fifth andsixth aspects a computer program may be combined with a music track thecombination being recorded on a recording medium for example a compactdisc.

These and other aspects of the invention are set out in the claims towhich attention is invited. The features of the claims may be combinedin combinations other than those explicitly set out in the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the present invention, reference will nowbe made by way of example to the accompanying drawings in which:

FIG. 1 is a schematic block diagram of a first illustrative system forprocessing audio signals in accordance with the present invention;

FIG. 2 is a schematic block diagram of a second illustrative system forprocessing audio signals in accordance with the present invention;

FIG. 3 is a schematic block diagram of a third illustrative system forprocessing audio signals in accordance with the present invention;

FIG. 4 is a schematic block diagram of a fourth illustrative system forprocessing audio signals in accordance with the present invention;

FIG. 5 is a schematic block diagram of a fifth illustrative system forprocessing audio signals in accordance with the present invention;

FIG. 6 is a schematic block diagram of a sixth illustrative system forprocessing audio signals in accordance with the present invention;

FIG. 7 is a schematic block diagram of a seventh illustrative system forprocessing audio signals in accordance with the present invention;

FIG. 8 is a schematic block diagram of an eighth illustrative system forprocessing audio signals in accordance with the present invention;

FIGS. 9 to 14 are illustrative data structures in accordance with theinvention;

FIG. 15 is an illustrative block diagram of an audio signal player inaccordance with the present invention;

FIG. 16 is a flow chart illustrating the operation of the player of FIG.15;

FIG. 17 is a schematic diagram of a networked system in which thepresent invention may be used;

FIG. 18 is a flow chart illustrating a method of distributing bits ofaudio signal amongst samples of audio signal; and

FIG. 19 is a flow chart of a method of reversing the process of FIG. 18.

In the Figures, like elements are denoted by like reference signs.

The present invention is described in the following by way of examplewith reference to audio signals.

Compression and Encryption of Audio Files

Referring to FIG. 1, a source 1 of original digital audio is provided.The audio in this example is in the form of a file. The digital audio isuncompressed in any suitable format, for example, PCM, WAV, AIFF (AudioInterchange File Format). The digital audio may be in fixed point formator in floating point format. The following description assumes fixedpoint format, although a section below comments on floating pointformat. The source may be any suitable source, and in this example maybe a computer. The original digital audio is compressed by a compressor2, which implements a standard lossless compression algorithm to producea compressed audio file. It will be appreciated that other compressionalgorithms may be used. The compression achieved by the compressor 2depends on the content of the audio. The compressed audio is encryptedby an encryptor 4 which implements a standard encryption algorithm whichdoes not significantly increase the size of the compressed audio file toproduce a losslessly compressed and encrypted audio file. A smallincrease in the number of bits may be acceptable. Such an increase maybe due to the provision of Cyclic Redundancy Code information forexample. Rijndael is one example of such an encryption algorithm: othersmay be used. The encryption algorithm uses at least one key. The key orkeys needed to decrypt the file are securely stored in a store 3 for usein decrypting the audio later.

Combining the First, Compressed and Encrypted, Audio Signal with aSecond Audio Signal

In the example of FIG. 1, the first signal, which is the compressed andencrypted audio, is combined in a combiner 8 with a second audio signalfrom a source 6. The encryption of the first signal from source 1creates a signal which appears to be noise to the human ear in thecombined signal because of the randomising effect of the encryption. Thesecond signal from source 6 is any audio signal. One example is anannouncement. An example of an announcement lets a user know what theoriginal audio from source 1 is and how they can obtain the key or keysfor decrypting the original audio. In other examples, described in moredetail below, the second signal is an impaired version of the originalaudio. In this example the second audio signal can be intelligiblyreproduced at least without decryption and preferably withoutdecompression even when combined with the first signal from source 1.

The combined signals, which in this example are a file, are then storedin a store 9, and/or provided to a transmission system 9, and/orrecorded on a suitable recording medium 9 indicated as distributionmaterial in FIG. 9. The transmission system may be a “pull” system forexample the Internet or a “push” system for example any broadcast systemincluding DAB (Digital Audio Broadcast) and DVB (Digital VideoBroadcast).

The example of FIG. 1 has been described as if the compressor 2,encryptor 4, and combiner 8 are discrete hardware elements. Whilst theymay be such, the system of FIG. 1 may be implemented in software foroperation on a computer.

FIG. 2 differs from FIG. 1 by showing details of an example of thesource 6 of the second signal. In this example the source 6 provides tothe combiner 8 an impaired version of the original audio from source 1.The impaired audio in this example is reproducible without decryptionand decompression when combined with the original audio, which appearsas noise in the combined signal. This allows a user to hear an auditionsignal, which is an impaired version of the original audio, withoutallowing them access to the original unimpaired audio.

Mixing a Spoiler Signal with an Audio Signal

As shown in FIG. 2, the source 6 preferably comprises a mixer 12 whichmixes original audio from source 1 with a spoiler signal from a source11 to produce an audition signal. The mixer is an additive mixer and theoutput of the mixer is the audition signal. The relative amplitudes ofthe original audio and the spoiler in the audition signal are a matterof judgement. If the level of the spoiler is too small with respect tothe original audio it may be encourage unauthorised attempts at removal.If its level is too high it will mask the original audio too much. Theaudition signal is combined with the compressed and encrypted originalaudio in the combiner 8 to produce the distribution material 9. Thespoiler signal may be any signal. An example of a spoiler signal is anannouncement as described above. The source 11 of spoiler signals maystore a suite of different signals which may be chosen according to theaudio to be spoilt.

Modulating the Spoiler Signal

FIG. 3 shows a modification of the system of FIG. 2, in which amodulator 14 is provided. The modulator 14 modulates the spoiler signalto make it more difficult to remove. The modulation may be any suitablemodulation including for example reverberation, phase modulation andpitch shifting.

FIGS. 1 to 3 show systems which allow the second signal from source 6,which may be an impaired version of the original signal, to bereproduced without the need for decompression and decryption. Thus inthe case of the second signal being an impaired version of the originalaudio from source 1, the audition signal has the same format (albeitspoilt) as the original audio and can be reproduced using any playercapable of reproducing the original audio from source 1.

Compressing the Audition Signal

FIGS. 4, 5 and 6 show systems which are modifications of the system ofFIG. 2. They differ from FIG. 2 by the addition of a compressor 16 inFIGS. 4 and 18 in FIGS. 5 and 6. The compressor 16 or 18 compresses theMSBs of the audition signal. The LSBs are removed before compressionbecause they are replaced by the first, compressed and encrypted audio,signal as described below. The system of FIG. 3 including the modulator14 may be modified in the way shown in FIGS. 4, 5 and 6. In FIG. 4, thecompressor 16 operates according to a lossless compression algorithm andmay be the same as compressor 2. In FIGS. 5 and 6, the compressor 18operates according to a lossy compression algorithm examples of whichare compression algorithms defined in the MPEG standards. In order toreproduce the audition signal, a reproducer needs a decompressorcorresponding to the compressor 16 or 18.

Combining the Compressed and Encrypted Audio Signal with the AuditionSignal

The combiner 8 of any one of FIGS. 1 to 5 may operate in the followingway.

Assume the compressed and encrypted audio signal is in the form of afile and the audition signal is also in the form of a file. Assume alsothat both files have the same format although that is not essential.Referring to FIG. 10, each file contains sampled audio, having samplesrepresented by digital numbers having Most Significant Bits (MSBs) andLess Significant Bits (LSBs). Each file has a Header which may forexample identify at least the type of file (e.g. WAV) and the number ofsamples in the file. The number of bits per sample may be any numberconventional in the art, for example 8, 16, 24 or 32. FIG. 11 describedbelow assumes 16 bits per sample P1, P2 . . . Pn.

A) In one example, the LSBs of samples of the audition file are replacedby samples of the compressed and encrypted audio signal. Thus in thecombined signal the MSBs represent the audition file and the LSBs arenoise representing the original audio from source 1 and occur in thecombined signal as noise.

If the audition file is sufficiently long, all the samples of theencrypted and compressed audio are placed in the LSBs of the auditionfile. If the encrypted and compressed audio file does not fill the LSBsof the audition file, then the unused LSBs of the audition file may bereplaced by zeros or they may be retained unchanged. If the encryptedand compressed file is longer than can be accommodated by the LSBs ofthe audition file, then additional samples of the combined file arecreated comprising zero filled MSBs and LSBs which are the samples ofthe encrypted and compressed audio.

The ratio of MSBs to LSBs in each sample of the combined signal file maybe fixed, or alternatively it may be variable. Referring to FIG. 11, atleast when the ratio is variable and preferably also when the ratio isfixed, data h is provided in the combined file indicating the boundarybetween the MSBs and the LSBs. For a fixed ratio, the data h may beprovided once in the file at the head of the file. It is added to theencrypted data represented by the LSBs as shown in FIG. 11. If the ratiois variable, the data h is provided at each position where the ratiochanges.

The ratio of MSBs to LSBs may be chosen as a function of the lengths ofthe file of the audition signal and of the file of the compressed andencrypted audio signal.

Bit Distribution

B) Referring to FIG. 18 another method (which is referred to herein as“Bit Distribution”) of distributing the bits of the compressed andencrypted signal over samples of the audition signal is as follows.

This explanation assumes that a file of audio is processed as shown inFIG. 2. The original audio has n-bit samples which are compressed bycompressor 2 and encrypted by encryptor 4 by an encryption algorithmwhich does not significantly change the number of compressed bits. Byway of example assume 10 seconds of stereo audio sampled at 48 kHz with16 bit samples and a compression ratio of 0.65. The number of auditionsamples is then 48000*2*10=960000. The number of compressed bits is0.65*960000*16=9,984,000.

At step S50 the ratio of the number of compressed bits to the number ofuncompressed audition samples is calculated. For this example the ratiois 10.4.

At step S51, that ratio is converted to a simple integer fraction M/Nequal to or greater than the said ratio, where M and N are low integers.10.4=52/5. Thus M=52 bits are to be distributed over each set of N=5samples as the LSBs of those audition samples. (In practice it isdesirable to choose a value of M which is less than the word length ofthe computer on which the method is implemented to keep the subsequentprocessing simple. M and N are stored (S511) to enable the subsequentreversal of the process.

At step S52 a value S=2^(R) is calculated. R equals M/N.

At step S53, a group of N=5 samples and M=52 bits is obtained. Headerdata is added to the bitstream to indicate the group. For the purposesof the following explanation assume the M bits represent an M bit numberof value V. The samples are ordinally numbered 0 to N−1, in this example0, 1, 2, 3 and 4.

At step S54 set X=N−1.

At step S55, A′[X]=((A*(F−S)/F)/S)*S is calculated for each of the Nsamples, where A is the range of the audition sample. This scales thevalue of A and is termed a scaled value below.

By way of explanation Step $55 is a combination of two sub-steps whichwhen combined produce step S55. In the first sub-step each audio sampleis pre-scaled according to A′[X]=A[X]*((F−S)/F) so it fits in the range−(F+S) . . . (F−S). F is the maximum value which a sample can take. Fora 16 bit sample F=+/−2¹⁵⁻¹. The second sub-step scales each pre-scaledaudio sample A′[X] according to A″[X]=(A′[X]/S)*S so that it has a valuein set{−(F+S), −2S, 1S, 0, 1S, 2S . . . F−S} . . .

At step S 56 replace the current scaled value A″ by a new valueA′[X]+V/S^(X) where A′[X] is the ordinally numbered scaled value. Thisadds the bits representing V/S^(X) to the scaled value A″[X].

At step S57, V is replaced by V−V/S^(X).

At step S58, if N is not 0 then N is decremented by 1 and another set ofbits of V are added to the next sample by steps S55 to 57. If N=0 thenthe 0^(th) sample is replaced by A′[0]+Vmod S which has the effect ofadding the remaining value of V to the 0^(th) sample.

Referring to FIG. 19, the method of extracting the M original bits fromthe combined bitstream is as follows.

Starting at step S62, the group of N samples A′ are obtained and at stepS63. X and V are set initially to 0. Then at step S64 the current valueof V is replaced by a new value V+(A′[X] mod S)*S^(X). Thus for thefirst calculation X=0 so V=A′[0]mod S.

If X is not N−1 at step S65, then X is incremented by one at step S66and V replaced by the new value at step S 64. That cycle repeats untilall N samples have been processed, the final value of V=V+A[4]modS*S⁴being the restored original bits.

C) Referring to FIG. 6 the encrypted material file is appended to theaudition file as is shown schematically in FIG. 13. Alternatively,encrypted material is inserted as a non-audio section in the combinedfile.

Reducing the Size of the Combined File.

It is possible that, for a given file-size of the combined file, whenusing the combining method A described above, the number of MSBsrepresenting the audition signal in the combined signal may be too smallto provide an adequate audition signal for a user to hear but it is notdesired to increase the size of the file. It is possible to make morespace available for the MSBs representing the audition signal, if theaudition file is a multi-channel file, by converting the file to asingle channel (mono) file. The compressed and encrypted audio may beplaced in the LSBs of the audition signal by any of the methodsdescribed above. Any encrypted audio which cannot be fitted into theLSBs of the audition file may be appended to the audition signal (seeFIG. 13), or inserted into the combined file as a non-audio section.

In one method, if the file has two channels, (i.e. it is a stereo file),each of 16 bits and 7 bits are needed for the audition signal, then thetwo channels may be converted to one mono channel of 32 bits. Thatincreases the bits available for the compressed and encrypted signalfrom 2*(16-7) [i.e. 18] to 1*(32-7)=25.

In another method, provided the number of bits per sample is greaterthan the number of channels the format of the original signal can bemaintained and each channel of the signal is replaced by a monofold-down of all the channels. As the mono signals are coherent, thishas the effect of making the audition content of the distribution fileperceptibly louder (for each added channel). The noise is incoherent, soremains at an unchanged perceived level. Consequently, an extra MSB inthe audition file may be made available to the encrypted material (foreach of the original channels) without loss of perceived quality exceptfor loss of the multi-channel audio image.

A further method produces a single mono representation of the originalsignal, reduced to the size of one of the original channels. Thisgreatly reduces the file size, allowing any excess encrypted data (afterthe merge operation) to be appended to this file (or inserted as anon-audio section).

A method for reducing the file-size of the combined file, which may beacceptable in some circumstances, is to reduce the sample-rate of thesamples in the audition file, using standard down-conversion algorithms.This has the effect of reducing the frequency range of the auditionsignal and also effectively decreasing the file-size of the auditionsignal relative to the original. Encrypted audio is placed in the LSBsof the samples as described above. Any encrypted audio which cannot befitted into the LSBs of the audition file may be appended to theaudition signal (see FIG. 13), or inserted into the combined file as anon-audio section.

Converting a multi-channel file to a single channel file as described inthis section may also be used in conjunction with the combining method B(Bit Distribution) described above,

MPEG Compression

Referring to FIG. 7, the compressor of the audition signal may be anMPEG compressor. An MPEG data structure has data space which may be usedfor auxiliary data and data space for the compressed audio as is shownin simplified and schematic form in FIG. 14. In this example, thecompressed and encrypted audio signal is placed in the auxiliary dataspace of the MPEG data structure.

Thus the audition signal can be reproduced using an MPEG reproducer.

Compression formats other than MPEG may provide equivalent auxiliarydata space. MP3 may be used for example with auxiliary data spacetherein.

In addition to, or as an alternative to using the auxiliary data space,the compressed and encrypted audio may be appended to the end of theMPEG data structure as shown in FIG. 13.

Techniques Using Blocks, Segments or Sections of Audio. PartialEncryption and Decryption.

In some situations it may be desirable to allow only sub-sections of theoriginal material to be extracted from the distribution material. Inthis case, as the original file is being compressed in compressor 2, itis split into segments of a fixed length (e.g. 1 sec, 5 sec, 10 sec), orinto sections at specified points. In the example of an audio track of afilm or video, the sections may correspond to scenes. In this example,different encryption keys are used to encrypt the different segments orsections, and saved in a lookup file for later decryption. In thisexample, the compressed/encrypted file must contain information atdeterminable points (normally section or segment boundaries), whichindicate which section or segment of the original audio is containedwithin that section or segment. This information is necessary, as thecompression obtained is not constant throughout the compression process,so there is not a fixed method for calculating position in the originalmaterial from position in the encrypted/compressed version. Thecompression/encryption in this mode can be done in one of severaldifferent ways, two of which are mentioned here:

-   -   1. The whole file is compressed in a single pass. This generally        produces the most efficient compression ratios, but requires        that section or segment headers are placed in the compressed        file during the compression process. The headers BH are placed        at block boundaries to indicate what part of the original audio        is contained in the following section or segment; see FIG. 12.

Sections or segments of desired length are then separately encrypted andmerged with the audition signal using a method as described above.

The sections or segments of the encrypted data may be organised asfollows:

-   -   a) Referring to FIG. 9, the sections or segments are based on        the header data BH inserted during compression. Section or        segment headers SH1, indicating the encrypted sections or        segments, are provided in the encrypted data. The headers        include location start data plus block length data, block ID        data and possibly other data. The location start data is        extracted from the compressed data headers BH during the        encryption process. This allows a section or segment to be        located without needing to decrypt the data.    -   b) Alternatively, section or segment header data is not included        in the encrypted data, in which case it is then necessary to        rely on the headers of the compressed data to locate a section        or segment. That requires decrypting at least some of the        segments to locate the headers in the compressed data.    -   2. The original file is split into the desired segments or        sections, each of which is compressed and encrypted separately.        The resulting compressed and encrypted sections or segments are        then concatenated as shown in FIG. 9. In this case, the segment        location information SH only needs to be embedded in the        encrypted file although it is not encrypted itself. The        compressed and encrypted file including the headers SH is then        combined with the audition signal (as in the methods described        above)

A modification which is applicable to both of the above segmentedencryption methods is to place a complete lookup table LUT ofsegment/offset information at a known place in the encrypted orcompressed file, which enables the correct segment or section to belocated quickly.

In this variant, decryption and extraction is similar to that describedin the method described above. When a valid request is received toextract a segment or section of the distributed file, the key(s) for thesegment or section is/(are) retrieved from the lookup file, the segmentor section is located in the encrypted data (LSBs) of the distributedfile, using the information saved in the block header(s), and thesections or segments decoded.

If a user requests the extraction of a portion of the audio which islonger than one segment or section but does not coincide with section orsegment boundaries, then whole sections or segments which include therequested portion are extracted to allow correct decryption anddecompression. However, more data is extracted than is requested. Thusblocks which contain audio samples outside the requested segment areextracted. These are discarded after decryption and decompression. Whenchoosing a basis for the original file segmentation, care should betaken to ensure that the decodable segments sizes and the permittedextraction request sizes do not allow large numbers of samples to bedecoded outside requested portions of audio, as they are potentiallyavailable for unauthorised use. This potential security gap iscompletely avoidable if permitted segment extraction exactly followsencoding lengths (eg on 10 second boundaries), or the original materialis explicitly encoded in segments and extraction requests are restrictedto those segments.

Adaptive Bit-Slicing.

Louder audio signals mask noise better than quiet ones. In this,adaptive bit-splicing, variant the signal level of the spoilt materialis analysed before or during the combining stage and is divided intoblocks with different average loudness. The blocks may be either offixed length, or of variable length determined by loudness thresholds.The sequence of blocks thus generated may then be combined with theencrypted data, using a bit-slice (MSB:LSB ratio) based on the loudnessof each block. In this method, the compression and encryption of thefirst signal may be performed on the whole of the original material, butthe encrypted data is also broken into blocks at the sameblock-boundaries as the audition material with header information at thestart of each encrypted block giving the number of samples at a givenbit-slice, which is required when the encrypted data is subsequentlyextracted when the original is restored.

This variant may be combined with the with the partial-decryptionvariant (above), by setting a maximum segment size for audio of similarloudness which allows decryption of the original on the requiredboundaries.

Streaming Audio.

In some situations, it may be required that the operation of creatingthe distribution file should start generating output before the wholeoriginal audio has been read. This might be the case if the originalaudio is received from a network connection rather than a file, or theoutput is being encrypted on-the-fly for distribution on a networkconnection. It is of course not possible to see the whole file beforecompressing or encrypting must start. The procedure to follow is similarto that for Partial Encryption-method 2 described above. The maindifferences are that: the same encryption key may be used for allblocks; a predetermined block length is used; and that it is not alwayspossible to generate a continuous bit-stream for the encoded version ofthe data, as compression ratios obtained will vary from block to block.There is a choice of strategies, influenced by the actual compressionratio obtained for each block of original audio data, and the fact thatthe generated data must not stall waiting for subsequent blocks.

-   -   a If the MSB:LSB split is fixed, and the compression achieved        does not permit the encrypted version to fit in the LSB stream        (even using all the techniques described above), then the extra        bits of the encrypted data remaining after the block of        distribution data has been packed, then the extra bits must be        placed at the head of the encrypted data of the next block    -   b If the MSB:LSB split is fixed, and the compression achieved        means that the encrypted version is smaller than the space        available in the LSBs of the distribution material, then the        remaining LSBs are filled with random noise.    -   c If the MSB:LSB split is variable, and the compression achieved        does not permit the encrypted version to fit in the LSB stream,        then either the MSB:LSB split is changed so that the all the        bits of the encrypted data can be accommodated, or (if a        practical limit to the split is reached), the extra bits of the        encrypted data are moved to the head of the next block.    -   d If the MSB:LSB split is variable, and the compression achieved        means that the encrypted version is smaller than the space        available in the LSBs of the distribution material, then the        remaining bits can be filled with random noise, or the MSB:LSB        split can be raised to allow more MSBs from the audition tone to        be heard.        Note that in cases c and d, it is possible that the SNR of the        distributed signal will vary from block to block if the MSB:LSB        split is changed. This is probably acceptable where the reason        for the good/poor compression was because the original signal is        quiet/loud in those blocks (louder noise is better masked by        louder signals), and is the preferred method, as adjacent blocks        generally obtain similar compression ratios.

The method described in the section Bit Distribution may also be appliedto streaming audio.

Floating Point Format

The preceding description assumes that the digital audio is representedin a fixed point format. However the digital audio may be represented infloating point format. Those skilled in the art will recognise thatfloating point format might result in more complex processing.

A Combined System (FIG. 8)

The invention has been described with reference to FIGS. 1 to 7 whichindividually show respective different methods of processing an audiosignal. The various methods described above may all be implemented byapparatus as shown in FIG. 8. FIG. 8 will now be described, but thedescriptions of the various methods described above which may beimplemented by the apparatus will not be repeated here.

FIG. 8 differs from the apparatus of FIG. 1 to 7 as follows:

The compression ratio achieved by the compressor 2 may be measured andif the compressor 16, 18, 181 of the audition signal is provided, thenthe compression ratio of that compressor 16, 18, 181 may be controlledin dependence on the measured compression ratio. That enables thecompression of the audition signal to be adjusted to provide anappropriate ratio of compressed MSBs representing the audition signal toLSBs representing the compressed and encrypted original audio for agiven file size. Alternatively or additionally, the manner in which theaudition signal is combined with the compressed and encrypted originalaudio signal may be chosen in dependence on the measured compressionratio, which also enables an appropriate ratio of MSBs (representing theaudition signal) to LSBs (representing the compressed and encryptedoriginal audio) for a given file size to be selected.

FIG. 8 also comprises a control panel 20 which may be a virtual controlpanel provided as a graphical user interface of a computer which enablesa user to set various operating parameters of the apparatus. The controlpanel may set one or more of the following:

a) The choice of spoiler signal if a suite of spoiler signals isprovided.

b) The relative signal levels of the spoiler signal and the signal beingspoiled.

c) The choice of modulation if a suite of modulations is provided asdescribed above.

d) Type of compression provided by the compressor 16, 18, 181 of theaudition signal. Various types of compression are described above.

e) Type of combination of the audition signal and of the compressed andencrypted original audio signal. Various types are described above.

f) The type of block or segment or section of audio. Blocks may be offixed length or variable. Segments or sections may be fixed or variable.They may be chosen to correspond to scene changes for example asdescribed above. The control panel may be used to designate the scenechange locations.

g) The control panel may be used to add information about the sectionsor segments.

h) Parameters for raw (headerless) PCM files such as sample rate, sampleformat etc.

The system of FIG. 8 may be controlled by an operator to select, andoperate in, one of the manners described above or may be so controlledby a computer program.

Reproducer

Referring to FIG. 15, there is shown schematically an example of areproducer (also referred to as a player). The combined file orbitstream is received at an input 50 and fed via a decompressor 51(corresponding to compressor 16, 18 or 181 described above) or directlyto a reproduction stage 54 which is operable to reproduce theuncompressed audition signal. A switch 53 selects the direct connectionto the input 50 or the decompressor 51 dependent on whether the auditionsignal is compressed or not.

If the reproducer is intended to also reproduce the compressed andencrypted audio, it further comprises a separator 55 which separates theencrypted and compressed audio from the audition signal, a decryptor 56which decrypts the separated signal using the keys 3, a decompressor 57and a reproducing stage 58 which reproduces the original audio.

Referring to FIG. 16, for a data structure as shown in for example FIG.11 or 12, the separator 55 parses S2 the bitstream and detects S4 theheaders to determine where the boundary between LSBs and MSBs lies. TheMSBs representing the audition signal are then discarded S6 leaving theencrypted and compressed audio represented by the LSBs.

If the first and second signals are combined by the method described inthe section “Bit Distribution” described above then they are separatedusing the method described with reference to FIG. 19.

The decryption key or keys are then obtained S8 and used to decrypt S10the LSBs. The decrypted audio is then decompressed S12.

For other data structures such as those shown in FIGS. 13 and 14, thecombined data structure is parsed S2 to determine the location(s) of theencrypted and compressed audio and the audition signal discarded S6.

Transaction Systems

Referring to FIG. 17, the present invention may be implemented as partof a system by which audio or any other data is traded. In FIG. 17 theterms “seller” and “buyer” are used for ease of description. Whilstthose terms may indicate their ordinary meanings implying that a vendorsells the audio to a buyer outright in return for payment, they are alsoused here more generally to indicate that audio may be made available bya person or organisation (the seller), who may be acting on behalf ofthe originator and/or owner of the audio, to another person ororganisation (the buyer) who may pay for the use of the audio underpredetermined business conditions but transfer of the ownership of theaudio does not necessarily occur.

Referring to FIG. 17, a first example of a system in accordance with theinvention comprises a transaction server 62, a seller client 60, a buyerclient 61 and a communications network 64 linking the clients to theserver.

The owner of material, i.e. the seller, controls the seller client 60. Abuyer controls the buyer client 61. In this example, a third party ownsand controls the transaction processor 62 although the transactionprocessor may be owned and controlled by for example the seller. Thesystem allows audio material to be acquired, securely and perceptiblyspoiled or impaired as described herein above, and transferred to thebuyer for the buyer to audition (69) the impaired audio material. If thebuyer then wants to buy the original unimpaired audio material, thebuyer obtains from the transaction server 62 the data needed to accessthe unimpaired audio. In this example, the seller and buyer bothregister (651, 652) with the transaction server. The data for accessingthe unimpaired audio is sent from the transaction server to the buyeronly when the buyer has paid for the material. The payment is monitoredby the transaction server 62 which communicates with a financialinstitution 63. Payment is made via the server 62 and/or via theinstitution 63.

Associated with the seller client 60 is a first apparatus 66 forimpairing the audio material as described above. The apparatus 66 may beas shown in any of FIG. 1 to 9 and 18 and 19. The key(s) needed fordecryption of the encrypted audio are transmitted from the apparatus 66to be stored in the transaction server 64 together with an identifierwhich identifies the material. Associated with the buyer client 61 is asecond apparatus 68 for accessing the unimpaired audio. Such anapparatus may be as shown in FIGS. 15 and 16.

A content auditioning apparatus 69 is also denoted in FIG. 17 inassociation with the buyer client 61 for the purpose of hearing theaudition signal. Such an apparatus may comprise elements 50, 51, 52, 53and 54 in FIG. 15 which enable the buyer to reproduce the auditionsignal.

The seller client and the buyer client may be computers which implementthe impairment of audio and access to the audition signal and theunimpaired audio. As part of the registration process, software forimplementing the impairment of original audio may be provided by theserver to the seller client. Likewise software for accessing theunimpaired audio may be provided by the server top the buyer client.

In this example the material is audio material and is recorded on arecording medium 9, e.g. a tape, disc or other store or is a bitstreamform an external source. The material is acquired and processed by thefirst apparatus 66. In addition the material identifier is applied tothe material. Then the material including the identifier is transferredon the medium 9 to the second apparatus 68, 69. The transfer is forexample by post. Alternatively, the audio material may be transferredvia the network 64.

The identifier is applied to the audio material during acquisition orduring processing of the material to enable the key(s) to be associatedwith the audio and to enable the buyer, seller and transaction server tomanage the selling and buying of the audio material. An example of anidentifier is a Unique Material Identifier or UMID. UMIDs are describedin more detail in SMPTE Journal March 2000.

To obtain the unimpaired audio, the buyer pays for the audio and obtainsthe decryption key(s) from the server 64.

A system as shown in FIG. 17 is described in more detail in EuropeanPatent Application published as EP-A-1215 907 which is incorporatedherein by reference.

In an alternative example, there is no transaction server and the sellercommunicates directly with the buyer via the network.

In another alternative example, both a seller client and a buyer clientmay be implemented on the same computer.

The invention may also be implemented in a peer to peer network.

In yet another alternative, the there is no transaction server 62. Thebuyer who has a buyer processor 61 communicates with the seller who hasa seller processor 60 via the network 64 to obtain the combined audiodata either via the network or an a tape or disc or other recordingmedium which is sent to the buyer. If the buyer likes the auditionsignal, the buyer pays the seller for the decryption key(s) eitherdirectly or via the financial institution 63 connected to the network.The seller then sends the decryption key(s) to the buyer.

Push Systems and Pull Systems

The transaction systems described with reference to FIG. 17 are “pull”systems in which the buyer requests the seller to transfer the auditionsignal to the buyer.

The present invention may be used in “push” systems an example of whichis a broadcasting system in which audio is transmitted to all potentialusers. If a user then wishes to acquire the unimpaired audio they thenrequest the issuance of the decryption key(s).

Computer Program Combined with an Audio Signal

In the foregoing examples, the first signal is a compressed andencrypted digital audio signal which is combined with a second audiosignal, which is the audition signal. In some examples, the encryptionrandomises the first signal so that it appears to be noise if it isembedded in the second signal. In other examples, the second signal iscompressed and the first signal embedded in auxiliary data space in thecompressed audition signal and/or appended to the compressed secondsignal.

In a development of the invention, the first signal is a computerprogram which may or may not be compressed and which may or may not beencrypted. This example assumes it is neither compressed nor encrypted.The second signal is an audio signal. The combined signal is recorded ona recording medium, for example a compact disc.

The computer program is:

-   a) embedded in the second signal (which may or may not be    compressed) according to one of the methods described above. In this    case the first signal need not be encrypted but preferably it is    encrypted. If the computer program is compressed it is losslessly    compressed.

Alternatively, the computer program is

-   b) embedded in auxiliary data space in the compressed second signal    and/or appended to the end of the compressed second signal according    to methods described above. Preferably it is encrypted especially if    it is simply appended to the second signal. If the computer program    is compressed it is losslessly compressed.

The second signal may be music. The second signal may possibly includean announcement making clear that a computer program is on the disc andgiving instructions on how to access it.

1. A method for processing a digital audio signal, the method comprisingthe steps of: providing a digital audio signal representing unimpairedaudio information; compressing and encrypting said digital audio signalto produce a first compressed and encrypted audio signal, the audioinformation of which is substantially unimpaired compared to that of thesaid digital audio signal; producing an unencrypted second audio signal;and combining said first and second audio signals to produce a combinedsignal comprising the compressed and encrypted first audio signal andthe unencrypted second audio signal, wherein said first audio signaloccurs as noise in said combined signal; and the step of combining thefirst and second signals comprises embedding the first signal as noisein the second signal.
 2. The method according to claim 1 wherein thedigital audio signal is losslessly compressed to produce the first audiosignal.
 3. The method according to claim 1, wherein the step ofcombining comprises appending at least part of the first signal to thesecond signal.
 4. The method according to claim 1, wherein the step ofproducing the second signal comprises impairing at least a portion ofthe digital signal.
 5. The method according to claim 3, wherein the stepof producing the second signal comprises combining the digital signalwith a third signal which impairs at least a portion of the digitalsignal.
 6. The method according to claim 3, further comprising the stepsof: modulating the third signal; and combining the modulated thirdsignal with the digital signal.
 7. The method according to claim 1,wherein the step of producing the first signal comprises: compressingthe digital signal; and encrypting the compressed signal withoutsubstantially increasing the number of bits of the compressed digitalsignal.
 8. The method according to claim 1, wherein the second signal isa sampled digital signal, each sample having more significant bits(MSBs) and less significant bits (LSBs).
 9. The method according toclaim 8, wherein the digital signal has a fixed point format.
 10. Themethod according to claim 8, wherein the first signal is combined withthe second signal by replacing the LSBs of the second signal with atleast some of the bits of the first signal.
 11. The method according toclaim 10, wherein a predetermined fixed number of LSBs of the secondsignal are replaced by at least some of the bits of the first signal.12. The method according to claim 10, wherein, in the combined signal, aratio of MSBs, representing said second signal, to LSBs, representingthe bits of the first signal, is variable.
 13. The method according toclaim 12, wherein the ratio is dependent on compression applied to thefirst digital signal.
 14. The method according to claim 10, wherein thecombined signal includes data indicating which bits of the combinedsignal are LSBs and which bits are MSBs.
 15. The method according toclaim 8, further comprising the step of reducing an amount of data inthe second signal.
 16. The method according to claim 15, comprising thestep of reducing a sampling rate of the second signal.
 17. The methodaccording to claim 8, further comprising providing a file containing thefirst signal and a file containing the second signal.
 18. The methodaccording to claim 17, wherein a ratio of MSBs, representing the saidsecond signal, to LSBs, representing said first digital signal, isdependent on a number of bits in the files of the first signal and anumber of bits of the second signal.
 19. The method according to claim17, wherein the bits of the first signal are distributed over samples ofthe second signal based on a ratio of the total number of encrypted bitsin the encrypted signal file to the total number of samples of thesecond signal.
 20. The method according to claim 19, wherein the ratiois approximated by an integer fraction M/N, and further comprising thesteps of: selecting groups of N samples; and distributing, over the Nsamples of each group, corresponding sets of M bits.
 21. The methodaccording to claim 20, further comprising the steps of: scaling a valueA of each of the N samples according to A′[X]=(A[X]/S)*S where: X is anordinal numbering of the samples and equals 0 to N−1; and S=2^(R) whereR is M/N; and replacing A′[X] by A″[X]=A′[X]+V/S^(X) for X>0, and byA″[0]=A′[0]+mod S for X=0, where for each of X=N−1 to 0, V is replacedby V-V/S^(X), V initially being the value of the M bits when X=N−1. 22.The method according to claim 1, wherein the second signal is a sampleddigital signal, each sample having most significant bits (MSBs) and lesssignificant bits (LSBs), and comprising the step of dividing the secondsignal into blocks each block comprising a plurality of samples.
 23. Themethod according to claim 22, wherein each block of the second signalcontains the same predetermined number of samples.
 24. The methodaccording to claim 22, further comprising the steps of: analysing thesignal level of the second signal; and setting the number of samples perblock based on signal level.
 25. The method according to claim 22,wherein the number of samples per block in the second signal varies. 26.The method according to claim 25, further comprising the steps of:analysing the signal level of the said second signal; and setting thenumber of samples in a block based on a function of the levels of thesignal samples within the block.
 27. The method according to claim 22,further comprising providing, in the second signal, data indicating theboundaries of the blocks.
 28. The method according to claim 27, wherein,in each block, the first signal is combined with the second signal byreplacing the LSBs of the second signal with bits of the first signaland a ratio of MSBs, representing said second signal, to LSBs,representing the bits of the first signal, in each block is a functionof the signal levels of the samples of the second signal in the block.29. The method according to claim 28, wherein the data indicating theblock boundaries includes data indicating the number of samples in eachblock.
 30. The method according to claim 1, wherein the step ofproducing the first signal further comprises the steps of: compressingand encrypting the digital audio signal, and wherein at least the stepof encrypting comprises: selecting sections of the compressed digitalaudio signal; separately encrypting each section; and providing data inthe first signal indicating the section boundaries.
 31. The methodaccording to claim 30, further comprising providing a file containingthe digital audio signal to be compressed and encrypted.
 32. The methodaccording to claim 31, further comprising the steps of: compressing anentirety of the file; and encrypting sections of the compressed file.33. The method according to claim 31, further comprising the steps of:selecting sections of the file; separately compressing and encryptingeach sections; and providing each section with data at least identifyingthe section.
 34. The method according to claim 30, further comprisingthe steps of: encrypting at least one section according to oneencryption key; encrypting at least one other section according toanother key; and storing data indicating the correspondence between thesections and the keys.
 35. The method according to claim 34, wherein thecorrespondence data is stored in the first digital signal.
 36. Themethod according to 30, wherein the data indicating the sectionboundaries identifies the data included in the sections.
 37. The methodaccording to claim 1, further comprising the step of compressing atleast part of the second signal and wherein the combining step comprisescombining the first signal with the compressed second signal.
 38. Themethod according to claim 37, wherein the compressed second signalcomprises auxiliary data space within the data structure thereof, andcomprising the step of placing at least some of the bits of the firstdigital signal in the said auxiliary data space of the compressed secondsignal.
 39. The method according to claim 37, wherein the second signalis compressed according to an MPEG standard.
 40. The method according toclaim 8, wherein the step of producing the first digital signalcomprises: receiving the digital signal from a streaming source;dividing the digital stream into segments each comprising apredetermined number of samples; and separately compressing andencrypting each segment.
 41. The method according to claim 40, furthercomprising: encrypting all sections according to the same key orencrypting at least one section according to one encryption key, and atleast one other section is encrypted according to another key; andstoring data indicating the correspondence between the sections and thekeys.
 42. The method according to claim 41, wherein the correspondencedata is stored in the first digital signal.
 43. The method according toclaim 40, wherein the first signal is combined with the second signal byreplacing, in samples of the second signal, the LSBs of the secondsignal with the bits of the first signal.
 44. The method according toclaim 43, wherein a predetermined fixed number of LSBs of a sample ofthe second signal are replaced by the bits of the first signal.
 45. Themethod according to claim 44, wherein, in samples of the combinedsignal, the ratio of MSBs, representing the second signal, to LSBs,representing the bits of the first signal, is variable.
 46. The methodaccording to claim 45, wherein the ratio is dependent on an amount ofcompression applied to the first signal.
 47. The method according toclaim 43, wherein the combined signal includes data indicating whichbits of the combined signal are LSBs and which bits are MSBs.
 48. Themethod according to claim 43, further comprising appending at least partof the first digital signal to the second signal.
 49. The methodaccording to claim 40, further comprising the steps of: selecting groupsof N samples; and distributing over the N samples of each groupcorresponding sets of M bits of the first signal, where the ratio M/N isan integer fraction.
 50. The method according to claim 49, furthercomprising the steps of: scaling a value A of each of the N samplesaccording to A′[X]=(A[X]/S)*S where: X is an ordinal numbering of thesamples and equals 0 to N−1; and S=2^(R) where R is M/N; and replacingA′[X] by A″[X]=A′[X]+V/S^(X) for X>0, and by A″[0]=A′[0]+mod S for X=0,where for each of X=N−1 to 0, V is replaced by V−V/S^(X), V initiallybeing the value of the M bits when X=N−1.
 51. The method according toclaim 1, further comprising the step of recording the combined signal ona recording medium.
 52. The method according to claim 1, furthercomprising providing the combined signal to a signal distributionsystem.
 53. The method according to claim 1, further comprisingproviding the combined signal to a transmission system.
 54. A recordingmedium including instructions that when run on a data processor causesthe data processor to implement the method of claim
 1. 55. A recordingmedium according to claim 54, wherein the recording medium includes oneof a tape and a disc.
 56. An apparatus comprising a processor programmedto execute the method of claim
 1. 57. A system comprising at least firstand second processors, the system being adapted to execute a method oftransferring a digital signal representing content from the firstprocessor to the second processor, the method comprising the steps of:using the first processor to implement the method of claim 1 to producethe combined signal and to associate an identifier with the combinedsignal for identifying the combined signal; storing the identifier;transferring the combined signal to the second processor; at the secondprocessor, deriving the identifier associated with the combined signal;transferring to the second processor, at least one key associated withthe said identifier, based on one or more predetermined conditions fordecrypting the encrypted first signal; and utilizing the secondprocessor to separate the first signal from the second signal and torestore the first signal.
 58. A system comprising a transaction serverand at least first and second clients, the system being adapted toexecute a method of transferring a digital signal representing contentfrom the first client to the second client, the method comprising thesteps of: using the first client to implement the method of claim 1 toproduce the combined signal and associating an identifier with thecombined signal for identifying the combined signal; providing, to thetransaction server, the identifier and at least one key for decryptingthe encrypted signal and storing, in the transaction server, theidentifier and the at least one key; transferring the combined signal tothe second client; deriving the identifier associated with the combinedsignal; transferring the identifier from the second client to thetransaction server; transferring from the transaction server to thesecond client at least one key associated with the said identifier,based on one or more predetermined conditions, for decrypting theencrypted first signal; and using the second client to separate thefirst signal from the second signal, and using the decryption keydecrypt the first signal, decompress the decrypted to restore thedigital signal.
 59. The system according to claim 57, wherein the firstsignal includes computer readable instructions.
 60. An apparatus forprocessing a digital signal, the apparatus comprising: a first input forreceiving a digital audio signal representing complete and unimpairedaudio information; a compressor and encryptor arranged to compress andencrypt the digital audio signal arranged to produce a compressed andencrypted first audio signal, the audio information of which issubstantially unimpaired compared to that of the digital audio signal; asecond input for receiving an unencrypted second audio signal; and asignal combiner arranged to combine the first and the second audiosignals to produce a combined signal comprising the compressed andencrypted audio signal and the unencrypted second signal, wherein: thefirst audio signal occurs as noise in the combined signal, and thesignal combiner is operable to embed the first signal as noise in thesecond signal.
 61. The apparatus according to claim 60, furthercomprising a first signal producer operable to produce the digital audiosignal representing unimpaired audio information.
 62. The apparatusaccording to claim 60, further comprising a second signal produceroperable to produce the unencrypted second audio signal.
 63. Theapparatus according to claim 62, wherein the second signal producerfurther comprises a signal impairing for impairing the digital audiosignal to produce said second signal.
 64. The apparatus according toclaim 63, wherein the second signal producer comprises a second combinerfor combining the digital audio signal with a degradation signal thatdegrades the digital audio signal to produce the second signal.
 65. Theapparatus according to claim 64, further comprising: a modulator formodulating the degradation signal, and wherein the second combiner isarranged to combine the modulated degradation signal with the digitalaudio signal to produce the second signal.
 66. The apparatus accordingto claim 65, wherein the second signal is a sampled digital signal, eachsample having most significant bits (MBSs) and less significant bits(LSBs) and wherein the signal combiner is operable to combine the firstsignal with the second signal by replacing the LSBs of the second signalwith bits of the first signal.
 67. The apparatus according to claim 66,wherein the signal combiner is arranged to control a ratio of the numberof LSBs to MSBs according to the compression ratio achieved by thecompressor.
 68. The apparatus according to claim 66, wherein the signalcombiner is adapted to append at least part of the first digital signalto the second signal.
 69. The apparatus according to claim 60, whereinthe signal combiner is arranged to distribute the bits of the firstsignal over samples of the second signal based on a ratio of the totalnumber of encrypted bits in the encrypted first audio signal to thetotal number of samples of the second signal.
 70. The apparatusaccording to claim 69, wherein the ratio is approximated by an integerfraction M/N where M/N is less than the ratio, and further comprisingthe step of: selecting groups of N samples and distributing over the Nsamples of each group corresponding sets of M bits.
 71. The apparatusaccording to claim 70, wherein the signal combiner is arranged toimplement the steps of: scaling a value A of each of the N samplesaccording to A′[X]=(A[X]/S)*S where: X is an ordinal numbering of thesamples and equals 0 to N−1; and S=2^(R) where R is M/N; and replacingA′[X] by A″[X]=A′[X]+V/S^(X) for X>0, and by A″[0]=A′[0]+mod S for X=0,where for each of X=N−1 to 0, V is replaced by V−V/S^(X), V initiallybeing the value of the M bits when X=N−1.
 72. The apparatus according toclaim 60, further comprising a second compressor operable to compressthe second signal, the signal combiner being arranged to combine thefirst signal with the compressed second signal.
 73. The apparatusaccording to claim 72, wherein the compression ratio of the secondcompressor is dependent on the compression ratio achieved by thecompressor.
 74. The apparatus according to claim 60, wherein thecompressor and encryptor are arranged to produce a losslessly compressedand encrypted first audio signal.
 75. A method of recovering a firstsignal from a combination of a first, compressed and encrypted, digitalaudio signal combined with a second signal, in which the audioinformation of the first audio signal is substantially unimpaired; thefirst audio signal occurs as noise when combined as a combination withthe second signal; the first audio signal is embedded as noise in thesecond signal; and the method comprises the steps of: separating thefirst audio signal from the combination; decrypting the separated firstsignal; and decompressing the decrypted first signal to recover thesubstantially unimpaired audio information.
 76. The method according toclaim 75, wherein the first signal is represented by Less SignificantBits (LSBs) of the combined signal and the second signal is representedby Most Significant Bits (MSBs) of the combined signal and furthercomprising the step of discarding the MSBs to separate the first signalfrom the second signal.
 77. The method according to claim 76, whereinthe first signal is appended to the second signal and further comprisingthe step of discarding the second signal.
 78. The method according toclaim 75, wherein the second signal is a compressed signal, compressedaccording to a format which has auxiliary data space in which the firstsignal is placed, and further comprising the step of extracting thefirst signal from the auxiliary data space.
 79. The method according toclaim 75, wherein the first and second signals are combined, wherein therecovering method comprises, for each group of N samples, the steps ofsetting X=0, setting V=0, and replacing V by V=V+A′[X]modS*S^(X) foreach of X=0 to N−1.
 80. A recording medium including instructions thatwhen run on a data processor implements the method of claim
 75. 81. Arecording medium according to claim 80, wherein the recording mediumincludes one of a tape and a disc.
 82. A processing apparatus comprisinga processor programmed to implement the method of claim
 75. 83. A methodof processing a digital signal comprising the steps of: providing afirst digital signal representing first information; providing a seconddigital signal; and embedding the first signal in the second signal byselecting groups of N samples and distributing over the N samples ofeach group corresponding sets of M samples of the first signal, wherethe ratio M/N is an integer fraction, further comprising the steps of:scaling a value A of each of the N samples according to A′[X]=(A[X]/S)*Swhere: X is an ordinal numbering of the samples and equals 0 to N−1; andS=2^(R) where R is M/N; and replacing A′[X] by A″[X]=A′[X]+V/S^(X) forX>0, and by A″[0]=A′[0]+mod S for X=0, where for each of X=N−1 to 0, Vis replaced by V−V/S^(X), V initially being the value of the M bits whenX=N−1.
 84. The method according to claim 83, wherein the second signalis an audio signal.
 85. An apparatus comprising a processor programmedto execute the method of claim
 83. 86. A non-transitory computerreadable medium having recorded thereon instructions that when run on acomputer or computer system, implements the method of claim
 83. 87. Anon-transitory computer readable medium according to claim 86 comprisingat least one of a tape and a disc.